Use of modifiers with metal atomizers in electrothermal AAS: a short review

The evolution of chemical modifiers for Mo and W atomizers is slowly progressing, based mainly on trial and error experimentation. Despite repeated use of some chemical compounds, such as thiourea for Mo atomizers, at this point there is no panacea similar to the Pd+Mg mixture used widely in graphite furnace atomic absorption spectrometry. Clearly, the chemical processes involved during atomization from a metal atomizer differ significantly from those occurring in a graphite tube, so successful graphite modifiers may not be readily adapted to metal atomizers. As a result, the analyst must begin anew with the evaluation of many potential modifiers in hopes of finally arriving at some universal solution. The purpose of this review, with 62 references, is to describe that journey to date, and to point out some promising paths that may lead to future success: such as the development of permanent chemical modifiers for W and Mo electrothermal atomizers.     

An ion exchanger-epoxy resin pelletization method for sample preparation in X-ray fluorescence analysis. Microanalysis of metal ions in industrial waste water

Flame atomic emission and atomic absorption detection limits are compared on an absolute basis for the easily determined element, copper. Remarkably similar detection limits are obtained with a variety of atomization and sample introduction system combinations. Heated graphite atomizers offer significantly lower minimum detectable quantities.     

Electrothermal atomic absorption spectroscopy - preseht understanding and future needs

A comparison is made between Massmann-type furnaces (with and without the L'vov platform) and constant temperature atomizers. It is shown that there is no major difference between these types of furnaces with regard to peak height sensitivities. On the other hand, the Massmann-type furnaces shoved to a greater extent susceptibilities towards matrix interference effects. The effect of the sample residence time on gas phase interference effects has been investigated at various constant temperatures for lead in large excesses of iron chloride and sodium sulphate, respectively. These experimental results are discussed and they are correlated to data obtained by high temperature equilibrium calculations. As a conclusion we found that there is a need for a better control of the gas phase inside graphite tubes. Advantages of separating the volatilization and atomization processes are discussed. The potentialities of constant temperature atomizers for atomic emission spectroscopy are lined out.Since its inception, conventional GFAAS has been developed considerably with regard to methodology and instrumentation. The technique has been essentially improved by the introduction of e.g., automatic sample devices, the L'vov platform technique, matrix modifications, pyrolytically coated graphite, automatic background correctors, adequate signal evaluation and rapid controlled heating of the atomizers. In spite of this progress there still remain problems in connection with the vaporization/atomization of samples. In conventional Massmann-type furnaces, the temperature at which an element is vaporized depends on its volatility and usually effective atomization temperatures are often too low for complete atomization. An additional disadvantage comprises difficulties in relating absorbance signals, which may originate from different atomization intervals, to true amounts of an element. Many of these problems inherent in Massmann-type furnaces can be eliminated by vaporizing samples into atomizers which are kept at a constant temperature. This concept was employed in the first graphite furnace ever built for analytical AAS [l], but due to the technical complexity of the isothermal approach, it has only been realized on a minor scale and therefore little is known about its limitations.By vaporizing samples from a platform [2,3] inserted into Massmann-type furnaces, the problems arising from non-isothermal atomization can often be minimized in a relatively simple way. In particular for volatile elements it is possible to approach conditions of constant temperature atomizers by the combined use of the platform technique with an element stabilizing modifier solution [4,5].The aim of this paper is to characterize isothermal as well as Massmann-type atomizers (equipped with and without platforms) with respect to sensitivity and susceptibility to interference effects as well as identifying future needs in order to develop the graphite furnace technique further.     

The determination of temperatures in a graphite tube atomizer with a conventional atomic absorption-flame emission spectrometer

Temperatures up to 3200°C in graphite tube atomizers can be measured by a two-wavelength intensity ratio method. The melting point of gold serves as a standard to calibrate the spectral response and the transmittance of the optics. The errors in the temperatures measured are 20–100° C. A conventional two-channel atomic absorption-flame emission spectrometer was used, but any single-channel instrument is also suitable for measurements. The method can be used for any type of heated graphite or metal flameless atomizer.     

Diode laser atomic absorption spectrometry

The paper reviews the past 11 years of literature on the application of diode lasers in atomic absorption spectrometry with graphite furnaces (GF), plasmas and flames as atomizers. Experimental arrangements and techniques for powerful absorption measurements as well as the theoretical background are covered. The analytical possibilities of high-resolution spectroscopy, including Doppler-free techniques for isotope selective measurements and isotope dilution analysis are discussed and various applications of element-selective detection by diode laser atomic absorption in combination with separation techniques, such as liquid (LC) and gas chromatography (GC), and with laser ablation of solid samples, are presented.     

Graphite atomizers modified with high-melting carbides for electrothermal atomic absorption spectrometry. II. Practical aspects

Data relating to practical applications of carbide-modified graphite atomizers (CMGAs) published mainly since 1985 are critically discussed. All elements being determined by means of electrothermal atomic absorption spectrometry are divided into ten groups according to their characteristics in graphite atomizers. CMGAs are the most effective for the determination of the elements forming rather stable oxides, such as Ge, Sn, Ga, In, as well as B and Si. The application of CMGAs to the determination of carbide-forming elements (Cr, Mo, V, Ti, etc.) may cause a significant decrease in their sensitivity. Concerning the high-volatility analytes (semi-metals and boron), CMGAs are usually effective only in the presence of regular chemical modifiers, primarily the salts of Pd and Ni. CMGAs may be successfully used for the determination of some organoelement (with Sn, Pb, Se, As) compounds, as well as for the trapping of volatile hydrides. CMGAs seem to be especially promising for analysis of biological samples, organic extracts, solid samples and samples containing high concentrations of mineral acids. High-melting carbides are prospective permanent modifiers.     

Development of electrothermal atomic absorption spectrometry in 2005–2016

The review covers publications of 2005–2016 on achievements in the development of electrothermal atomic absorption spectrometry (ETAAS). The main directions in the authors’ opinion are revealed, i.e., (1) improvements of the method and equipment and (2) studies of thermochemical processes in a graphite furnace. In the first group, the authors consider high- and low-resolution continuum source atomic absorption spectrometry, diode laser atomic absorption spectrometry, new designs of electrothermal atomizers, and new devices for ETAAS. Studies of mechanisms of element atomization, formation of analytical signals, and action of chemical modifiers belong to the second group.     

Electrothermal atomization—the way toward absolute methods of atomic absorption analysis

Considering the currently prevailing opinion that electrothermal atomizers are very susceptible to matrix interferences, an opinion which is contradictory to the optimistic forecasts of the first publications, the general status of the problem to date has been investigated.The first part of the paper deals with the ideal models of the two basic types of electrothermal atomizers, viz., those of the semi-enclosed and those of open type. The graphite cuvette and the Massmann furnace have been selected for discussion in their two commercial versions—the HGA and the CRA—of the first type of atomizers; the West filament and the combined atomizers—rod-in-flame and capsule-in-flame—of the second type. The models describing the distribution of atoms in the absorbing zone have been compared and the data obtained have been used to interpret the experimentally observed differences in the sensitivity of real atomizers.The second section of the paper discusses the conformity of real atomizers relative to ideal models and to the requirement of correctly recording the absorption by means of integrating the pulses. A marked time and space non-isothermality of the commercial atomizers has been established which makes it impossible to measure the integral absorption correctly. The graphite cuvette and the combined atomizers best meet the requirements of the ideal models. On the basis of the data obtained, the possible ways of bringing the semi-enclosed atomizers closer to ideal models have been explored. In this connection, the possibility of using a graphite platform for vaporizing samples in tube furnaces as well as using the temperature-controlled furnace and the pulse heated graphite furnace with a capacitance power supply has been examined. The application of the last method ensures time and space isothermality of the absorbing layer and reduces by 10–100 times the electric power input.The third section of the paper examines the thermochemical laws governing the possible chemical effects arising from the interaction between the analytes, on the one hand, and the furnace walls, the gaseous atmosphere and the matrix, on the other. A critical review has been carried out of the results of some recent publications on investigation of sample vaporization in graphite furnaces, which reveals the fact that the temperature of absorption appearance is rarely connected with the heat of vaporization of the free element. In most instances it is determined by the sum of the heat of decomposition of the non-volatile carbide and the heat of vaporization of the free element. Thermochemical examination of the stability of the compounds formed in the gaseous phase revealed the fact that besides forming monoxides, there is the possibility of forming monocyanides. In addition, the presence of large quantities of halogens causes partial binding of the analyte as gaseous monohalides, mainly monochlorides. In order to eliminate the latter effect, it has been suggested to employ higher atomization temperatures or to bind chlorine in a stable lithium monochloride. The efficiency of the proposed methods has been tested by experiment.Our research has shown that the main reasons for the unsatisfactory status of the problem concerning the matrix effects, are connected with the use of the amplitude (peak) method of recording the absorption, with the time and space non-isothermality of the absorbing layer of the commercial atomizers, and with the formation of gaseous monohalides. All these problems may be eliminated on the basis of theoretically proved and tested methods, some of which are discussed in this paper.     

Graphite filter atomizer in atomic absorption spectrometry

Graphite filter atomizers (GFA) for electrothermal atomic absorption spectrometry (ETAAS) show substantial advantages over commonly employed electrothermal vaporizers and atomizers, tube and platform furnaces, for direct determination of high and medium volatility elements in matrices associated with strong spectral and chemical interferences. Two factors provide lower limits of detection and shorter determination cycles with the GFA: the vaporization area in the GFA is separated from the absorption volume by a porous graphite partition; the sample is distributed over a large surface of a collector in the vaporization area. These factors convert the GFA into an efficient chemical reactor. The research concerning the GFA concept, technique and analytical methodology, carried out mainly in the author's laboratory in Russia and South Africa, is reviewed. Examples of analytical applications of the GFA in AAS for analysis of organic liquids and slurries, bio-samples and food products are given. Future prospects for the GFA are discussed in connection with analyses by fast multi-element AAS.     

Time resolution of interferences in electrothermal atomic absorption spectrometry

A fast detector-amplifier-readout system is used for studying interferences in electrothermal graphite atomizers. The effects of different matrix components (K, B, Ca, Mg, and Cl), and graphite tube surfaces significantly alter the atomization processes of lead.     

Diffusion vapour transfer modelling for an end-capped atomizer. Part 1. Atomizer with closed injection port

For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (T_R) of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%. The theoretically predicted T_R values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained T_R values for the endcapped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding T_R ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1–5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).     

Tungsten coil devices in atomic spectrometry: absorption, fluorescence, and emission.

In this work, tungsten coil (W-Coil) devices are used as atomizers for electrothermal atomization atomic absorption spectrometry (ETAAS), electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS), and electrothermal vaporization inductively coupled plasma atomic emission spectrometry (ETV-ICP-AES). For most cases in ETAAS and ETA-LEAFS, limits of detection (LODs) using the W-Coil are within a factor of ten of those observed with commercial graphite furnace systems. LOD for Cd by W-Coil AAS is 10 pg, while LODs for As, Se, Cr, Sb and Pb by W-Coil LEAFS are 950, 320, 1400, 330, and 160 fg, respectively. The compact W-Coil device makes it an ideal atomizer for portable atomic spectrometry instrumentation, especially when coupled with a miniature charge coupled device spectrometer. Alternatively, the atomizer can be used as an inexpensive, modular add-on to an existing commercial ICP-AES system; and the thermal separation of Pb with interference elements Al, Mn, and Fe is demonstrated.     

Computer simulation of two-step atomization in graphite furnaces for analytical atomic spectrometry

The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

Untersuchungen zur bestimmung seltener erden durch atomabsorption mit elektrothermischer atomisierung

The determination of rare earths by atomic absorption spectrometry with electrothermal atomizationThe electrothermal atomization of traces of rare earths has been investigated with different atomizers (carbon rod, graphite furnace, tantalum ribbon). The best analytical results are obtained with a modified tantalum thermal atomizer, because the formation of rare earth carbides is then impossible. Mixed argon—hydrogen atmospheres improve the concentration of atoms in the plasma, because hydrogen reduces the rare earth oxide radicals. The optimal analytical conditions are described. The detection limits are: 25 pg Yb, 22 pg Eu, 62 pg Tm, 2000 pg Sm, 300 pg Ho, 300 pg Dy, 1300 pg Er.     

Investigations of reactions involved in electrothermal atomic absorption procedures: Part 11. A theoretical and experimental investigation of factors influencing the determination of tin

Factors of importance for the determination of tin by graphite-furnace atomic absorption spectrometry have been investigated. Losses of tin during the drying and ashing stages were monitored by use of radioactivity measurements. It was found that tin could be lost at 100°C when ordinary graphite or glassy carbon surfaces were used. However, if the sample was dispensed into a droplet of ammonia, no losses occurred until 800°C, independent of the graphite surface, because tin is stabilized as SnO₂ (s,l) and chloride is removed as ammonium chloride. High-temperature equilibrium calculations indicate that tin forms volatile oxides, halides and sulphides which are stable up to relatively high temperatures. As is shown experimentally with commercial non-isothermal atomizers, these molecules might be removed from the system before their decomposition temperature is reached. A recently developed constant-temperature furnace was used to vaporize the sample into an environment of high and constant temperature, in which interference effects from chlorine and sulphur were minimized, as predicted by theory.     

Shadow spectral imaging of absorbing layers in a transversely heated graphite atomizer: Part 2. Molecules and condensed-phase species

The dynamics of formation and dissipation of chloride, nitrate and sulfate matrix vapors in a transversely heated graphite tube atomizer (THGA) with and without integrated platform was investigated with the use of multi-channel atomic absorption spectrometry and the shadow spectral imaging technique. It is shown that non-uniform heating of the tube walls and platform in the furnace radial cross-section causes vapor transfer from atomizer bottom to less heated sides of the tube and platform. This transfer in the atomizer cross-section can be an additional reason for lower level of matrix interferences in the THGA and is a prerequisite for explosive atomization of some elements that appear as absorbance spikes. The cross-sectional structures of molecular layers and the cloud of condensed phase particles are highly inhomogeneous, resulting in absorbance gradients up to 0.2–0.5 mm^− 1. These structures differ significantly from those observed earlier in end-heated atomizers. Local vortices of the sheath gas, toroid-shaped and bridge-like structures of vapor layers were observed in the atomizer volume. The role of light scattering on the finally dispersed condensed phase particles in the transverse heated furnace is greater than that in the end heated atomizers because of near axis location of the cloud.     

Matrix modification by Cu(NO3)2 for the determination of gold by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer

Matrix modification by copper nitrate in electrothermal atomic absorption Spectrometry (ETAAS) of gold with a molybdenum tube atomizer has been investigated. The addition of copper nitrate served to eliminate the interferences from 10⁴–10⁵-fold concentrations of foreign elements at the 890 °C pyrolysis temperature. The absolute characteristic mass (giving 0.0044 absorbance) of gold in the presence of copper nitrate with the Mo atomizer was 0.26 pg and the detection limit was 38 pg/ml. These values were several times better than those obtained with graphite atomizers. The recovery of spiked gold in biological materials was in the range 96–106%. A sensitive and accurate ETAAS method was developed for complex matrix samples.     

Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry

Atomization of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metal-carbidized surfaces.